Process for inhibiting the rusting of steel sheet and product obtained thereby



Patenteil Apr. 9,11946 PROCESS FOR INHIBITING THE RUSTING OF STEEL SHEET AND PRODUCT OBTAINED manner Charles B. Durg'hnAnniston, Ala., assignor to Monsanto Chemical Company, a corporation of Delaware No Drawing. Application September Serial No. 410,044

13 Claims. (Cl. 148-65) This invention relates to a process for treating steel sheets with phosphoric acid. In earlier known processes for treatment of steel with D phoric acid, the object has been to obtain a highly durable rust-proof surface. This was obtained by treating the metal with phosphoric acid solutions of various strengths, the treatment being continued for a length of time necessary to obtain a visible, gray to black crystalline deposit of various iron or other phosphates upon the surface thereof. The older processes resulted in the deposition of from 0.2 gram to 0.3 gram of P205 per square foot of surface, the P205 being combined as iron or other metal phosphates. The effectiveness of such treatment in many cases was in-. creased by the inclusion in the phosphoric acid used of various metals such as iron, zinc; chromium, copper, manganese and aluminum. These metals were generally precipitated as phosphates upon the surface of the iron or steel undergoing treatment to produce a rust resisting coating.

An objection to such coatings for some uses was that they are generally dark colored, the metal having lost its bright lustre.

It is one object of the present invention to provide a process for the rust inhibition of steel sheet of the type used for container manufacture, without the deposition of visible metallic phosphates upon the surface thereof. It is another object to provide a process for treating cold reduced steel sheet in order to retain the natural bright metallic lustre or appearance of such sheet.

It is a further object to provide a product comprising cold reduced sheet which carries a substantially invisible adherent,- elastic, protective film of colorless and transparent iron phosphate upon the surfaces. A still further object is to provide a phosphate-bearing surface upon cold reduced sheets upon which lacquer coatings, enamels, lithographic preparations, etc., may be applied with better adherence than upon untreated cold reduced sheets.

I have discovered that steel sheets, such as cold reduced sheets, which is likewise known as black plate, may be protected against rusting by the application of from 0.015 to 0.040 gram of P205 per sq. ft. of surface, applied as orthophosphoric acid and later combined as an iron-phosphoric acid reaction product. Usually from 0.02 to 0.04 gram of P205 per sq. ft. is a suitable amount and I have obtained excellent results with from 0.025 to 0.037 gram of P205 per sq. ft.

The phosphoric acid is applied at such strength,

which strength is correlated with the action of sired amount of P205, as HaPO4, is allowed to remain as a thin continuous film of acid upon the surface of the sheet. The film of acid is then reacted by the drying of the surface, usually by the application or heat, and without the removal of any of the acid or the further addition of acid thereto.

The sheets thus obtained, will be found to possess a bright lustrous surface upon which the development of rust is greatly delayed. By first employing suitable cleaning and scale removing processes, such as pickling, etc., the herein described process may also be applied to hot reduced tially pure commercial phosphoric acid to which no iron, manganese, chromium, aluminum, zinc, copper, arsenicor lead have been added.- For the purpose of retaining the lustrous or bright surface, it is desirable that .the iron content of the acid be restricted below a limit where excessive deposition of crystalline iron phosphates occurs during the time the sheet is in the acid bath.

In the carrying out of my process, immersion of the sheet is of such short duration that sub-. stantially no action of acid upon the steel surface occurs while in the bath; hence there is substantially no build-up of iron in the acid from this source. There may, however, be present upon the surface of cold reduced sheet minute particles of iron or iron oxide, which particles are rapidly dissolved by the acid.

In order to obtain a uniformly slightly etched lustrous surface, I find that it is sometimes desirable to first wash thestee'l sheets in a solution of trisodium phosphate at a temperature near the boiling point and for a limited time. Such a treatment serves to remove oil or grease spots from the sheets and thus permits the phosphoric acid treatment to uniformly wet the steel surface.

The following example will serve to illustrate the operation of my process and also the results the squeeze-rolls later described, so that the'dcobtained thereby:

A 30 /2-gaug'e cold reduced steel sheet was immersed in a boiling dilute (i. e., 3% to 5%) solution of trisodium phosphate for about one second;

then removed, rinsed in a body of clear boiling water and thereupon immersed for one second in a 25% solution of H3P04 maintained at C. The steel sheet was then removed from the acid, the surplus acid adhering to the steel sheet removed by passing the sheets between compressed stainless steel or rubber-covered rolls and thereafter directly into heated air so that the temperature of the sheet is raised to between 115 C. and 210 C. The slight deposit of acid upon the surfaces of the sheets was somewhat tacky at first, but the tackiness gradually decreased and was entirely eliminated after standing a short time at elevated or room temperatures.

The purpose of the compressed rolls is to remove any excess of phosphoric acid over that stated above as adhering to the sheets and at the same time to permit a very thin, uniform, non-flowing, adherent film of acid to be retained thereon. The film of acid thus retained upon the sheet thereafter loses water, becoming more con centrated and at the same time reacts with the iron surface. The course of reaction of the acid with the iron may be noted by the decreasing tackiness of the sheet. The reaction of the acid is accelerated by the heat of the drier and by this means may be considerably hastened. In some cases, however, because of the heat remaining in the metal, it may be possible by merely allowing the sheet to remain in air at room temperature, during which time normal drying takes place, to effect complete reaction of the acid with the iron of the surface, without application of extraneous heat. The results are somewhat similar, whether or not heated air drying is employed. In order to maintain the heat in the sheet, when employing hot air drying, it is also possible to stack the warm sheets in a tin plate box," even though the surfaces thereof are still somewhat tacky, and to permit the reaction consuming the acid on the surface to continue in such box without harmful results.

The conditions as given in my example above are preferred; however. it is possible to depart somewhat therefrom without sacrificeof results obtained. In general, I prefer to so correlate the related factors of time of immersion, temperature and strength of acid so that the treated sheet shows only a slight etch visible as a slight graying of the surface. Although the strength of the acid is not critical when correlated with the factors of time of immersion and the temperature of the phosphoric acid bath, I have found it desirable to employ a -50% or a -50% phosphoric ,acid solution. For most satisfactory results, however, I prefer to employ a phosphoric acid solution having a concentration between 15% and HaPO4. In order to obtain the desired results with varying acid strengths, within the above-mentioned limits, the time of immersion of the sheet in the acid is regulated within the limits of from 0.1 second to 1 or 2 seconds, the longer time being required with the weaker acid and vice versa. Temperature also is a factor of importance, increasing temperatures making for shorter time of immersion, decreasing temperatures for longer time. The preferred temperature is in the neighborhood of 75 C. but I have found also that temperatures between 50 C. and 85? C. and preferably between 65 C. and 80 C. may be employed. The P205 is believed to be combined in a transparent, colorless, invisible microcrystalline form with the underlying iron.

I have found that the rate of wettingof the metal surface can be somewhat accelerated by the use of a phosphoric acid solution containing a small amount, say in the neighborhood of 0.02% of dodecyl benzene sodium sulfonate. This sulfonate is an acid-soluble wetting agent and considerably accelerates the rate of wetting of the metal with the acid and reaction therewith and hence makes it possible to considerably decrease the time of ex osure of the metal sheet to t acid. I have also found that the etching effect is more uniform and considerably better rustproofing and a brighten-more lustrous surface is obtained thereby.

However, because of the decreased surface tension of the acid when a surface-active agent is employed therein as described, ordinarily a considerably smaller amount of acid (or P205) remains adhering to the surface, than when the same strength of acid is employed without such agent. Accordingly, it is desirable, when employing these wetting agents, to employ a suitably stronger acid than is employed without such agent, in order that the film of acid retained will contain the requisite amount of P205.

As a result of the present process it has been possible to produce cold-reduced or black plate which may be shipped by the producer without serious rusting during transportation and storage. In the past it has been the practice in many cases to employ various oils, lacquers and other protective coatings for the temporary protection of such products. When such coatings are employed it is necessary for the fabricator to remove such coatings by means of solvents, alkali washes, or by burning. Such processes are relatively expensive in that a separate operation and additional material are required. Moreover. such temporary protective means are not effective for inhibition of rust development, and the fabricator in the past has been forced to reject large quantities of rusted sheets. This practice has resulted in high cost and excessive loss of time and material to the industry.

The present process supplies a product which gives efllcient protection against rusting for a short period of time only, that is a period of time which may be from 30 to days, which is generally suflicient for the movement of cold reduced sheet from the steel mill to the fabricator, including reasonable storage periods. The herein described treatment is however ineffective upon sheets which are to be exposed to the elements.

It is of value, however, when such sheets are shipped and stored in the tin-plate boxes" employed in the industry. Thus my invention provides a tin-plate box as a stage product in the manufacture, transportation and storage of this product by the industry.

The rust resisting qualities of my sheet may be tested by an accelerated exposure test. This test comprises the wrapping of the individual treated sheets in kraft paper, after which the wrapped sheets were placed in a moisture saturated atmosphere at a temperature of 40 C. for 21 hours and thereafter in an ice refrigerator for three hours. It is considered that one week of exposure, utilizing this repeated cycle, is equivalent to one month of exposure during normal storage and shipment of steel sheets.

Upon testing my sheets, treated as above, I found that little or no rusting of the surface took place under these accelerated conditions, over a period of six weeks, whereas an untreated sheet rusted very badly. Moreover, the sheet treated according to my process retained their bright, lustrous, slightly etched surface which was particularly adaptable for the reception of paints or lacquers.

WhatI claim is:

1. The proces for inhibiting the rusting of a cold reduced steel sheet which comprises wetting said sheet with a substantially pure phosphoric acid solution, removing said sheet from said 60'? lution and thereupon passing it through queeze rolls to remove excess acid, correlating the temperature and concentration of said acid with the action of the squeeze rolls and also the time of contact of said acid with said sheet prior to passage through said rolls to form upon said steel sheet a uniform adhering film of said'acid containing between 0.015 and 0.040 gram of P205 per square foot of surface and thereupon react ing said film of acid with said steel sheet to form a substantially invisible elastic paint-, lacquerm and enamel-bonding coating which imparts rustinhibiting properties to said steel sheet without substantially impairing its natural bright metallic lustre.

2. The process for inhibiting the rusting. of a cold reduced steel sheet of the type used for container manufacture which comprises immersing said sheet for a period of from 0.1 to 2 seconds in a substantially pure phosphoric acid solution, re-

tially invisible elastic, paint-, lacquer. and enamel-bonding coating of combined PzOs which imparts rust inhibiting properties to said sheet without substantially impairing its natural bright metallic lustre, said coating containing between 0.015 and 0.040 gram of P205 per square foot of ,surface.

enamel-bonding coating consisting of an iron moving said sheet from said solution and thereupon passing it through squeeze rolls to remove excess acid, correlating the factors of time of immersion, temperature and concentration of the solution with the action of said squeeze rolls to form upon said steel sheet a uniform adhering film of said acid containing between 0.015 and 0.040 gram of P205 per square foot of surface and thereupon reacting said film of acid with said steel sheet to form a substantially invisible elastic paint-, lacquerand enamel-bonding coating which imparts rust-inhibiting properties to said steel sheet without substantially impairing its natural bright metallic lustre.

3. The process defined in claim 2 wherein the steel sheetis immersed in a 10 to 50% aqueous solution of HSPO'I.

4. Th rocess defined in claim 2 wherein the steel sheet is immersedin an aqueous solution of phosphoric acid containing a sulfonated wetting agent. i

5. The process defined in claim 2 wherein the phosphoric acid solution is maintained at a temperature of from to 85 C.

6. The process for inhibiting the rusting of a 7 solution 'and passing it through squeeze rolls to 50 remove acid in excess of from 0.015 to 0.040 gram of P201; per square foot of surface and thereupon d ying said sheet to cause said acid to react with said metal sheet to form a substantially invisible elastic paint-, lacquerand enamel-bonding coating which imparts rust-inhibiting properties to said sheet without substantially impairing its natural bright metallic lustre.

7. The process defined in claim 6 wherein the 8. A cold reduced steel sheet of the type used in container manufacture carrying a substancold reduced steel sheet of the type used in conphosphate which imparts rust inhibiting propertie to said steel sheet without substantially impairing its natural bright metallic lustre, said coating containing between 0.025 and 0.037 gram of P205 per square foot. of surface.

11. The process for inhibiting the rusting of a cold reduced steel sheet which comprises immersing said sheet for a period of from 0.1 to 2 seconds ina bath consisting of an aqueous solution of H3P04, removing said sheet from said bath and thereupon passing it through squeeze rolls to remove excess acid, correlating the factors of time of immersion, temperature and concentration of the bath with the action of the squeeze rolls to form upon said steel sheet a uniform adhering film of said acid containing between 0.015 and 0.040 gram of P205 per square foot of surface and thereupon reacting said film of acid with said steel sheet to form a substantially invisible elastic paint-, lacqlle'rand enamel-bonding coating which imparts rust inhibiting properties to said steel sheet without substantially impairing its natural bright metallic lustre.

12. The process for inhibiting. the rusting of a cold reduced steel sheet which comprises immersing said sheet fora period of from 0.1 to 2 seconds in a bath consisting of an aqueous solution of H3PO4, removing said sheet from said bath and passing it through squeeze rolls to remove excess acid, correlating the factors of time sheet is dried at a. temperature between 115' and no of immersion, temperature and concentration of the bath with the action of the squeeze rolls to form upon said steel sheet a uniform adhering film of said acid containing between 0.025 and a 0.037 gram of P205 per square foot of surface and thereupon reacting said film of acid with said steel sheet to form a substantially invisible elastic paint-, lacquer and enamel-bonding coating which imparts rust inhibiting properties to said impairing its steel sheet without substantially natural bright metallic lustre. A

13. The processdeflned in claim 2 wherein the phosphoric acid solution is maintained at a temperature of from C. to 0.

CHARLES B. DURGIN. 

